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The field of the present invention is control systems and, more particularly, systems and methods for effecting control of a Stirling cycle cryocooler as well as the control of high temperature superconducting thin film filter subsystems.
Recently, substantial attention has been devoted to the development of high temperature superconducting radio frequency (RF) filters for use in, for example, mobile telecommunications systems. However, such filters are extremely temperature sensitive. By their very nature, high temperature superconducting (HTSC) materials are temperature dependent. At temperatures above their xe2x80x9ctransition temperatures,xe2x80x9d the materials behave like an insulator, and at temperatures below the transition temperature, the materials become superconducting.
Further, when a HTSC film is fabricated into a RF filter, temperature fluctuations stemming from kinetic inductance of the filter may have a substantial effect upon the operation of the filter and, in particular, upon the center-frequency of the filter. Similarly, fluctuations in temperature may have a substantial impact upon certain non-linear behavior characteristics of HTSC thin film filters. While the non-linear behavior characteristics of a HTSC thin film filter may have a relatively mild effect upon filter operation at temperatures below the transition temperature, the same cannot be said for the kinetic inductance effect. Further, as the temperature of operation of a HTSC thin film filter approaches, for example, the transition temperature of the filter, relatively minor fluctuations in the operating temperature can have very significant effects upon filter operation. Stated somewhat differently, as HTSC thin film filter systems are operated closer and closer to their respective transition temperatures, more and more care must be taken to control the temperature of the operating environment. Thus, it will be appreciated that HTSC thin film filter systems must be maintained at stable operating temperatures if proper operation of the systems is to be maintained. This is particularly so where HTSC filters are to be operated at or near their respective transition temperatures.
Those skilled in the art also will appreciate that increased temperature stability generally is required when more xe2x80x9cnarrow-bandxe2x80x9d filters are utilized within a HTSC filter system. The reason for this is that relatively small changes in operating temperatures (e.g., +/xe2x88x921xc2x0 K) may have a substantial impact upon the range of filter operation, particularly if a filter is operated at or near its transition temperature. Indeed, such changes in operating temperature may cause the center frequency of a HTSC filter to vary by as much as 100 kHz.
Now, because maximum advantage may be obtained through the use of HTSC thin film filters when the filters are operated in a narrow-band mode at approximately the transition temperature, those skilled in the art will appreciate that it is highly desirable, if not essential, to maintain very precise control of the operating temperatures of HTSC thin film filter systems.
Those skilled in the art also will appreciate that, when multiple HTSC filters are disposed, for example, within the dewar of a cryocooler, and the cryocooler is mounted, for example, on a telecommunications tower, substantial temperature control issues may arise. Simply put, a tower-mounted cryocooler will need to provide more lift (i.e., more xe2x80x9ccoldxe2x80x9d) on a hot afternoon than would be required on a cold night. Further, as the ambient temperature of the environment within which a HTSC filter system is mounted varies, temperature gradients will result between the system cold source (i.e., the cold finger of the system cryocooler) and the cold stage or location where the HTSC filters are located. In addition, with respect to tower mount applications, there is a conflict between the need to have the fastest possible cooldowns to the point of maximum allowable temperature on the heat reject of the cryocooler while at the same time, not overdriving the cryocooler.
U.S. Pat. No. 6,098,409 (xe2x80x9cthe ""409 patentxe2x80x9d), which is incorporated by reference herein, discloses a multi-stage temperature controller that includes first and second control loops. The first loop is used to regulate the cold finger temperature of, for example, a Stirling cycle cryocooler. The second loop is used to set a reference for the first loop based upon a comparison between a reference signal and a signal received from a cold stage temperature filter. Analog circuitry, for example, as shown in FIGS. 2-6C in the ""409 patent is employed to control the cryocooler temperature of the HTSC thin film filters.
Analog-based control systems for controlling the cryocooler drive and temperature of the HTSC filters, while useful, have several drawbacks. First, the analog technologies employed increases the overall cost of the device. Since the analog-based circuitry requires a large number of parts, reliability of the device is reduced to a certain extent since there are more parts which could potentially fail. Along these same lines, the increased parts count contributes to additional weight for the product. In addition, the analog-based design makes it difficult to change control parameters and algorithms for new products, or, alternatively, makes it difficult to improve upon the performance characteristics of existing products. Finally, analog circuitry changes its characteristics with respect to time (i.e., the age of the device) and temperature whereas digital-based control systems are more stable.
A need exists for a digital-based control system for control of the cryocooler drive and the temperature of the HTSC filters. The system preferably has a reduced parts count which advantageously reduces the overall weight and cost of the system. Similarly, overall reliability of the device is improved through the implementation of a digital-based control system. The DSP-based control system advantageously has a tighter temperature control algorithm that allows for the construction of sharper HTSC filters (i.e., filters which differentiate one frequency from another, where adjacent frequencies are progressively more close to one another), and reduces the need for filter margin in the filter specification, which allows for even higher performance systems.
In a first aspect of the invention, a temperature and drive controller includes a cryocooler drive loop for controlling the cryocooler drive in response to a measured cryocooler driving current and a cryocooler driving current set point. A temperature control loop is provided and generates the cryocooler driving current set point in response to either a cooldown profile algorithm or a comparison between a measured temperature and a set point temperature stored in DSP memory. The temperature and drive control loops are implemented using a digital signal processor.
In a second separate aspect of the invention, a HTSC thin film filter system for use with a Stirling cycle cryocooler having a temperature and drive controller includes a Stirling cycle cryocooler having a cold finger, a heat-sink including a plurality HTSC thin film filter substrates mounted thereon in micro-enclosures, the cold finger of the Stirling cycle cryocooler mating with the heat-sink, at least one micro-enclosure temperature sensor, and a temperature and drive controller according to a first aspect of the invention.
In a third separate aspect of the invention, a method of controlling the temperature of a cryocooler cold finger that is used to regulate the temperature of a HTSC filter system cold stage includes the steps of measuring the temperature of at least one temperature sensor, inputting a signal corresponding to the measured temperature of the at least one temperature sensor to a temperature controller, comparing the signal corresponding to the temperature of the at least one temperature sensor to a signal corresponding to a set point temperature, and outputting a digital value corresponding to a set point cryocooler driving current to a cryocooler drive controller based on the comparison. Next, the cryocooler driving current is measured, and a signal corresponding to the cryocooler driving current is input to the cryocooler drive controller. The signal corresponding to the cryocooler driving current is compared with the digital value corresponding to the set point cryocooler driving current and a cryocooler driving current is output to a cryocooler based on the comparison between the cryocooler driving current with the digital value corresponding to the set point cryocooler driving current.
In a fourth separate aspect of the invention, the cryocooler drive loop of the first aspect is used with a temperature control loop that generates the cryocooler driving current set point in response to a cooldown profile algorithm stored in DSP memory.
In a fifth separate aspect of the invention, the cryocooler drive loop of the first aspect is used with a temperature control loop that generates the cryocooler driving current set point in response to a comparison between a measured temperature and a set point temperature.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for controlling the drive and temperature of a cryocooler for use with HTSC filters. The apparatus has fewer components since the control is implemented using a digital signal processor-based control system. The apparatus preferably has tighter temperature control than previous analog-based designs. In addition, the apparatus can be updated with new parameters, software, and control algorithms to improve its performance characteristics. Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.